drm/msm/sde: assign virtual planes to RECT1 of sDMA

SDE driver validates pipe priorities for planes staged for source split.
Always assign RECT_1 of smart DMA pipes to virtual planes. It allows
the kernel clients to configure planes as per their priorities while
mapping planes in source split mode.

Change-Id: Ibb498e8f2e87c596c3d4c79c2d7e4dc20bc82f44
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>